Poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHB-HHx) and methoxy poly(ethylene glycol) (MPEG) blends were prepared using melt blending. The single glass transition temperature, T-g, between the T(g)s of the two components and the negative chi value indicated that PHB-HHx and MPEG formed miscible blends over the range of compositions studied. The Gordon-Taylor equation proved that there was an interaction between PHB-HHx and MPEG in their blends. FTIR supported the presence of hydrogen bonding between the hydroxyl group of MPEG and the carbonyl group of PHB-HHx. The spherulitic morphology and isothermal crystallization behavior of the miscible PHB-HHx/MPEG blends were investigated at two crystallization temperatures (70 and 40 degrees C). At 70 degrees C, melting MPEG acted as a noncrystalline diluent that reduced the crystallization rate of the blends, while insoluble MPEG particles acted as a nucleating agent at 40 degrees C, enhancing the crystallization rate of the blends. However, no interspherulitic phase separation was observed at the two crystallization temperatures. The constant value of the Avrami exponent demonstrated that MPEG did not affect the three-dimensional spherulitic growth mechanism of PHB-HHx crystals in the blends, although the MPEG phase, such as the melting state or insoluble state, influenced the crystallization rate of the blends. (c) 2006 Wiley Periodicals, Inc.